This invention relates to employing a laser beam to vaporize or otherwise alter a portion of a circuit element on a silicon substrate and is particularly applicable to vaporizing metal, polysilicide and polysilicon links for memory repair.
Semiconductor devices such as memories typically have conductive links adhered to a transparent insulator layer such as silicon oxide, which is supported by the main silicon substrate. During laser processing of such semiconductor devices, while the beam is incident on the link or circuit element, some of the energy also reaches the substrate. Depending upon the power of the beam, length of time of application of the beam, and other operating parameters, the silicon substrate can be overheated and damaged.
Laser processes of this kind have typically been conducted at wavelengths of 1.047 μm or 1.064 μm. Silicon has sufficiently low absorption at these wavelengths that the amount of beam energy employed to evaporate typical polysilicide and polysilicon links has not harmed the underlying silicon substrate.
It has been recognized, e.g., by the present inventor, by Lapham et al. in U.S. Pat. No. 4,399,345, and by others, that in laser processing of semiconductor devices, it can be advantageous to use wavelengths beyond the “absorption edge” of silicon (i.e., wavelengths greater than about 1.1 μm, where the absorption of silicon drops precipitously). This makes the silicon substrate more transparent to the laser beam, and reduces heating of the silicon caused by absorption of the beam. The preferred wavelength generally mentioned for this purpose has been 1.32 or 1.34 μm, though a broad theoretical range of low absorption has been identified. The 1.32 or 1.34 μm wavelength has been proposed as it is close to the minimum absorption of silicon.
Removal of memory links for memory repair is an application in which these considerations are relevant. As noted in “Computer simulation of target link explosion in laser programmable redundancy for silicon memory” by L. M. Scarfone and J. D. Chlipala, 1986, p. 371, “It is desirable that laser wavelengths and various material thicknesses be selected to enhance the absorption for the link removal process and reduce it elsewhere to prevent damage to the remainder of the structure.”
The usefulness, in general, of thicker insulative layers underneath links or circuit elements, and the usefulness of limiting the duration of heating pulses has also been recognized, as in the paper of which I was author, “Laser Adjustment of Linear Monolithic Circuits”, Litwin and Smart, 166/L.I.A., Vol. 38 ILAELO (1983).
Makers of semiconductor devices typically continue production of earlier developed products while developing and entering production of more advanced versions that typically employ different structures and processes. Many current memory products employ polysilicide or polysilicon links while smaller link structures of metal are used for more advanced products such as the 256 megabit memories. Links of 1 micron width, and ⅓ micron depth, lying upon a thin silicon oxide layer of 0.3 to 0.5 microns are being used in such large memories. Production facilities typically have lasers and related equipment capable of operating at the conventional wavelengths of 1.047 μm or 1.064 μm and also wish to have lasers and related equipment capable of operating in the wavelength region recognized for its lower absorption by silicon.